As is known, in electronic devices, the term "noise" indicates a random fluctuation in currents or voltages at the device terminals, and may seriously limit the minimum signal level that can be handled by the device.
The noise in each device is due to various physical causes, some of which have been known for some time. Of particular interest are what are known as "flicker" noise (also indicated 1/f) and "burst" noise, the first of which exists in all and the second in a significant percentage of devices.
Flicker noise is commonly acknowledged to be caused by fluctuations in the number of carriers, due to entrapment of the carriers in surface layers of the device, i.e., to tunneling at the semiconductor-oxide interface. According to accepted theory, the carriers in the semiconductor may communicate with trap levels at a given distance within the tunnel oxide layer, and remain trapped for some time prior to being re-emitted. In the case of transistors, in particular, flicker noise sources are located at the base-emitter junction.
Flicker noise is especially undesirable in the case of operational amplifier input transistors and audio preamplifiers.
Burst noise, on the other hand, is caused by a sharp variation in current between two or more constant values. Variation frequency may be very low (less than 1 Hz) or high (hundreds of Hz), in which case, burst noise may be confused with a high degree of flicker noise. This type of noise is generally attributed to the presence of defects, metal inclusions and precipitates in the space charge region of the junction; and the fluctuation in current depends on the extent, if any, to which the defect participates in conduction. The fact that burst noise is reduced by deficiency-reducing processes, such as gettering, would appear to bear out this theory.
In the case of flicker noise, the noise power at the output terminals of a transistor is proportional to I.sub.B.sup..alpha., where I.sub.B is the base current and .alpha. a constant ranging between 1 and 2. In the case of burst noise, the output noise power is inversely proportional to the square of the gain of the transistor, so that, for a given collector current, high-gain transistors are less affected by flicker and burst noise as compared with low-gain types.
One technique for reducing flicker and burst noise is to produce extremely high-gain (super-beta SBT) transistors with a gain typically ranging between 1000 and 10,000 for collector currents below 1 .mu.A. In NPN type super-beta transistors, the base layer is narrower as compared with standard transistors, for improving the base transfer factor (reducing recombination of the charge carriers) and so increasing gain. In a planar process, in particular, by reducing the base width (thickness of the P type base layer between the N.sup.+ type emitter region and the N type epitaxial layer) to roughly 0.2-0.3 .mu.m (as compared with the normal 0.8 .mu.m), current gain increases to as much as 2000-5000 (as compared with 200-300).
High-gain NPN transistors may be produced simultaneously with conventional NPN types by adding a photolithography and diffusion step to the planar process. In practice, following base diffusion of the conventional NPN transistor, windows are opened photolithographically, through which the emitter of the high-.beta..sub.F transistors is predeposited and partially diffused. This is followed by photolithography and diffusion of the conventional emitter, so that the emitter of high-.beta..sub.F transistors is deeper than that of conventional ones, thus considerably reducing the thickness of the active base (by way of comparison, refer to FIGS. 1 and 2 relative to a standard and a high-gain transistor, in which w.sub.B and w.sub.B, indicate the respective base widths).
High-gain transistors of the aforementioned type, however, present an extremely low (roughly 3 V) open-base collector-emitter breakdown voltage (BV.sub.CEO), and cannot be employed in applications requiring a higher voltage, due to the risk of punch-through between the emitter and collector.
Moreover, prediffusion increases the likelihood of emitter pipes being formed, due to penetration of an emitter portion inside the base region as far as the collector, as a result of crystallographic defects. As is known, such pipes so modify the I.sub.C -V.sub.CE output characteristics of the transistor that they become resistive. This problem is particularly felt in the case of power integrated circuits, even to the extent of eliminating the advantages of high gain, or resulting in production rejects and, hence, reduced efficiency.
A further drawback lies in the need for providing an additional masking step as compared with the standard fabrication process, thus increasing fabrication costs.
Another known noise reducing solution consists in producing transistors with a high A.sub.E /P.sub.E ratio, i.e., a high ratio of the area and perimeter of the emitter region visible from above, so as to reduce the contribution of the surface portions with respect to that of the deep regions (bulk). This solution, however, only provides for reducing flicker and is ineffective as regards burst noise.